A variable displacement axial piston pump having a rocker cam pivotally mounted in a rocker cradle within the housing may employ a fluid motor to change the displacement of the device. In one type of device vanes mounted on each side of the rocker cam project into sealed fluid chambers which cooperate with the vanes to make hydraulic motors. Fluid introduced into chambers defined on one side or the other of the vanes causes the rocker cam to pivot in the rocker cradle to change the displacement of the pump. A manual control for such a device may include a rotating control arm having a shoe which slides on the surface of a valve plate. The valve plate may have a pair of fluid receiving ports connected to fluid passages leading to the fluid receiving chambers on opposite sides of the fluid motor vane. Movement of the control arm in one direction or the other provides fluid to the vane chambers on one side or the other of the vane to cause the rocker cam to pivot to a position set by the control arm. Since the valve plate pivots with the rocker cam the control has an automatic follow-up feature. Such a manual control known in the art as a rotary servo type input control is described in detail in U.S. Pat. No. 3,967,541 assigned to the predecessor in interest of the Assignee of the present invention.
In the aforementioned rotary servo manual input control the fluid must be supplied to one set of vane chambers to bias the vanes in one direction and simultaneously fluid must be exhausted from the vane chambers on the other side of the vane to enable the fluid motors to operate. In fact, when pressure fluid from the shoe in the manual control handle is supplied to one port in the vane plate fluid simultaneously is exhausted from an uncovered fluid port connected to the opposite vane chamber. Thus, it may be seen that the low pressure or tank port for the device is internal to the pump.
The variable displacement axial piston pump described above may have the basic manual rotary servo input control supplemented with an automatic control system which destrokes the pump when fluid pressure or flow exceeds a predetermined set maximum. The same control increases the stroke of the pump when the fluid pressure or flow falls below the amount that has been set by the manual control. Such an automatic control system also assigned to the predecessor in interest of the Assignee of the present invention is described in detail in U.S. Pat. No. 3,908,519.
In the aforementioned manual rotary servo control device, the fluid ports in the valve plate connected to the vane chambers are uncovered when the pump is at a set displacement. Additionally, when the automatic control device operates to destroke the pump because of an excessive flow or pressure control fluid is supplied to the vane chambers through fluid passages other than those utilized by the manual input control. When this occurs pressure fluid flows out of the uncovered ports in the valve plate. Because of this, the fluid ports or passages contain orifices or are sized so as to minimize leakage it is apparent that if the leakage from the ports in the valve plate can be prevented that response of the pump to the automatic compensation system would be increased markedly. Additionally, the fluid passages in the valve plate and valve stem could be enlarged so that the pump would respond faster to the manual control.
Additionally, it has been found necessary to block the ports in the valve plate when the pump displacement is being controlled by an auxiliary device such as an electrically operated control valve which supplies fluid to the vane chambers of the fluid motors to change the displacement of the pump through an auxiliary set of passages and the manual rotary control device is made inoperative. If the ports are not sealed the auxiliary device cannot operate to change the displacement of the pump inasmuch as it utilizes servo fluid having a relatively low pressure to control the pump and the manual control uses the same fluid. An example of a hydraulic circuit where an auxiliary device supplies pressure fluid to fluid motors to change the displacement of a pump may be seen in U.S. Pat. No. 3,381,624 assigned to the predecessor in interest of the Assignee of the present invention.
Accordingly, it is desirable to provide a control for a variable displacement axial piston pump of the rotary servo type in which the fluid ports and passages which are supplied pressure fluid from a manual rotary servo input control to operate fluid motors to change the displacement of the pump are blocked when the rotary servo manual input control is not being operated to change the displacement of the pump.